Merge pull request #4619 from kolyshkin/1.2-4616

[1.2] libc/int/userns: add build tag to C file

Author: AkihiroSuda

libcontainer/internal/userns/userns_maps.c renamed to libcontainer/internal/userns/userns_maps_linux.c

@@ -1,3 +1,5 @@
+//go:build linux
+
 #define _GNU_SOURCE
 #include <fcntl.h>
 #include <sched.h>